# Pure SR, Relativistic Mass, and its Gravity?

Discussion in 'Physics & Math' started by Neddy Bate, Jun 5, 2018.

1. ### Neddy BateValued Senior Member

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You have restated what I have come to understand is your position, but you have not addressed the following points:

1. pervect himself uses a "dropped mass" to measure the acceleration
2. pervect himself states that the dropped mass is just an instantaneously co-moving inertial observer
3. If spaceship B is always co-moving with the belt, (as you agreed), then their instantaneously co-moving inertial observers are identical at all times
4. Thus they would have to measure the same acceleration
5. Thus pervect's method does not support, and even contradicts, your claim that spaceship B and the belt would measure different accelerations
6. And pervect's method does support my claim that spaceship B and the belt would measure the same acceleration

I do not have an account at that site, and am not interesting in making one. But if you do, and you would like to follow up with pervect, I would always welcome more information with an ever open mind. As I see it, you two disagree on that particular matter, with respect to post #50 in that thread.

I understand the appeal of just having this end, due to the ongoing effort it requires. But that would be the easy way out, considering all of these unanswered questions.

Last edited: Jun 14, 2018

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3. ### Q-reeusValued Senior Member

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So far so good. But then...
Wrong. The two masses being weighed are subject to different 'g' forces. For the reasons given ad nauseum. One thing I have not introduced earlier that may help is that of gravito-magnetism. On second thoughts best left out of this. The entire system in each case has to be considered. In one there is relative motion between source and mass, in the other not. That makes all the difference. It's why 'superman' in that #50 gets to see the gamma squared amplification.
Both 5 & 6 are wrong. See above.
No disagreement. Same final result in each situation considered. Just different methods of attack.
I really am tired of this, so yes, let's end it here and now. Try elsewhere if this continues to bug you. Places like Quora afaik allow questions to experts, without requiring registration. Cheers.

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5. ### Neddy BateValued Senior Member

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I can only surmise that you reject pervect's approach of using a "dropped mass" to measure the acceleration? Where the dropped mass is the instantaneously co-moving inertial observer??

You agree that spaceship B is always co-moving with the belt. I suggest you take account of their instantaneously co-moving inertial observers. Are they identical at all times, or not?

Note well that the first postulate of SR is that the physical laws of nature are the same in every inertial frame of reference.

Sorry you're tired of this. If its any consolation, its not so much fun for me either. I don't think I should have to pry agreement on the first postulate out, like a dentist pulling teeth.

Last edited: Jun 14, 2018

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7. ### Q-reeusValued Senior Member

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No I don't reject his approach there. You have misunderstood it's application. Remember, superman sees that gamma squared boost! As was the case using different approaches in earlier posts there.
If by that you mean would a stone dropped from rest in spaceship B, vs from rest in moving-belt-in-rocket, accelerate at the same value a (as determined in the rocket B and belt frames respectively!), the answer is no. A gamma squared factor differentiates.
Indeed. And mostly we have been considering pov in accelerated frames, which iirc as stated early on, is quite capable of being analyzed using SR. As is the case from an inertial frame pov.
Check out that #49 there again and note pervect and I agree on all your treadmill scenarios. Forget your twin spaceships case - it's perpetually confusing you. And please DO take the treadmill scenario elsewhere e.g. Quora. Once you keep getting the same final verdicts (assuming someone competent there replies), it may sink in.
Indeed!

8. ### Neddy BateValued Senior Member

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Then you do reject pervect's approach of using a "dropped mass" to measure the acceleration. Because his dropped mass is specifically the instantaneously co-moving inertial observer of spaceship B, which is always co-moving with the belt.

9. ### Neddy BateValued Senior Member

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Imagine two people of equal rest mass. Let one be in spaceship B, and the other be on the belt. Since we have agreed spacehip B and the belt are co-moving, that means they are always co-located.

The two people can interact normally, they can shake hands, they can give each other gifts, whatever.

Now let them both be standing on spring scales, at rest under their feet.

Do you really want to claim one scale registers four times the other in that situation? Something must be re-evaluated.

10. ### Q-reeusValued Senior Member

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Neddy - keep believing that if you insist. But then you will have to explain why pervect and I agree on all your belt scenarios!
Please - argue your case elsewhere, and after all the chaff is filtered out, see if a different outcome presents. If it does whoever gives such there is wrong. Period.

11. ### Q-reeusValued Senior Member

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The factor of 4 is specific to a gamma =2. Anyway yes your thinking needs re-evaluating. Both people are restrained to be at rest in their respective frames. Why cannot one feel different g force to the other?
Which must be the case - SR demands it. How many times does that have to be repeated? pervect and I both get the same final results. Therefore either both of us are equally wrong - or you are.

12. ### Neddy BateValued Senior Member

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Clearly pervect never said that two observers who are co-moving and co-located would measure the gamma squared difference in weight.

Last edited: Jun 14, 2018
13. ### Neddy BateValued Senior Member

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co-moving means their relative speed is ZERO

Last edited: Jun 14, 2018
14. ### Neddy BateValued Senior Member

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Not their "respective frames," they are at rest with respect to each other in all frames. That is what "co-located at all times" means.

Because they are at rest with respect to each other?

15. ### Q-reeusValued Senior Member

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OK true but I was making sure you didn't slip into using the 'lab' frames as reference in each ship. And further, in each frame, they 'see' a different value of source acceleration. Really!
No, more to it. See next post.

16. ### Q-reeusValued Senior Member

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Co-moving is insufficient information (and I take note of your #70). We also need to know whether the proper-frame source of a or g in each case has a relative lateral motion. How many times now has that point been made?
Look, take the plunge, sign up at PhysicsForums - today! Go to that thread or start another one and get pervect's ear there. Be a little daring. But also be prepared to change your whole frame of thinking on how SR really works and is properly applied. Bye.

17. ### Neddy BateValued Senior Member

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Maybe I will sign up there when I have time. I'm kind of lazy that way, lol...

If I did, I think pervect would agree with me that co-moving and co-located (at all times!) is sufficient information, since he uses instantaneously co-moving and co-located in his method of assessing acceleration / weight. It is the first postulate of SR after all.

It's probably just that you and I are using different methods of reasoning. No hard feelings. Good bye.

Last edited: Jun 14, 2018
18. ### Neddy BateValued Senior Member

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Also, if you look at pervect's post #50 in that other thread, it is clear that he is comparing two frames which have relative motion, as they have two different clock rates:

But if spaceship B and the belt have no relative motion, then they would have the same clock rate. Which is just one more reason to conclude they would find the same acceleration.

It is important to note that prevect is comparing an accelerating frame to its instantaneously co-moving inertial frame. It may be the case that the belt's instantaneously co-moving inertial frame measures the belt to have 4 gravities, but then it would also have to be the case that spaceship B's instantaneously co-moving inertial frame measures spaceship B to have 4 gravities. But we have already agreed spaceship B weighs the test mass as its proper weight, which is also what I suspect happens on the belt. Makes sense?

Last edited: Jun 14, 2018
19. ### Q-reeusValued Senior Member

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Sigh. No Neddy, take note that the true analogy has point on belt = superman. You agree superman weighs the mass at 4x what stationary observer there does? Then so it is for scales on belt vs scales still in lab frame. Stop trying to prove 1 = 4. It never computes. DO sign up at PF today! Do it! Give them grey hairs for a change.

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20. ### Neddy BateValued Senior Member

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I understand the explanation you've given me, which is that the lab frame feels 1 gee while the belt frame feels 4 gees, because in that case the rocket engine was physically attached to the lab frame, and therefore the belt and the source of acceleration (the rocket engine) are moving laterally relative to one another.

If that is the correct explanation, then so be it. But with the belt and spaceship B co-located at all times, and still experiencing different accelerations, then I think we have to reject pervect's "falling mass" measurement of acceleration. I don't see how it would give different results for the belt and spaceship B.

21. ### Q-reeusValued Senior Member

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Neddy, if you check back through all relevant earlier posts, note I always agreed there was co-movement (but not 'forever' - unless the treadmill is infinitely long!), but never agreed there was co-location. Co-location implies they share the same rocket/spaceship! But they are in different spaceships. Hence cannot be co-located. There must be a physical lateral displacement. Moreover the two spaceships (sources of a) have relative x-axis velocities - equal to the belt speed wrt lab frame in fact. Ponder that logical necessity.

22. ### Neddy BateValued Senior Member

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When I say co-located I simply mean that they are at the same x coordinate (the direction of lateral movement) over some extended finite time, and they are at the same y coordinate (the direction of accelerated upward movement) over some extended finite time. But of course there must be some space between them on the z axis, which we can make arbitrarily small.

Now ponder this: An observer standing on the belt drops a test weight. It should not matter if he drops it directly over the belt so that it lands on the belt, or if he drops it from a z coordinate which is off the edge of the belt in the z direction. If he drops it off the edge of the belt, he can drop it through a window in spaceship B, so that it lands in spaceship B.

Would you really want to hang your hat on the concept that the test weight falls according to 4 gees if it is over the belt, but magically falls according to 1 gee if it is over the edge of the belt?

23. ### Q-reeusValued Senior Member

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Thanks for making that point. I had been wrong in agreeing with your premise they would remain co-moving. Not so. As the two spaceships have relative x-axis motions, each will see the other's y-axis acceleration reduced by a factor of 4 wrt their own lab-frame a. So, while at some initial 'co-located' instant, a mass-on-scales on belt and mass-on-floor in spaceship B are co-moving, at any other instant there is relative vertical i.e. y-axis motion owing to the acceleration differentials. Which differential is made non-reciprocal owing to the belt frame having x-axis motion wrt it's housing rocket.
There - now we have a harmonious picture where everyone *should* agree a felt on belt is 4x that on floor in spaceship B. Happy with that now?